1. Field of the Invention
The present invention relates to a method of modifying low frequency components of a digital audio signal.
2. Prior Art
When music is performed live using large public address loudspeakers, low-frequency vibrations (below approximately 50 Hz) are generated. These low frequency vibrations travel through the floor and are mechanically coupled to listeners in the audience, causing them to sense the vibrations through their feet and also within the chest cavity. Engineers in recording studios, when mixing music must create a musical balance such that all frequency ranges (low, mid and high) can be reproduced on a wide range of loudspeakers. When this mixed music is listened to at home, or in another setting. it is likely to be using much smaller loudspeakers than those used for live performances. These smaller loudspeakers are often inadequate at reproducing low (i.e., bass) frequencies, and this sometimes leads to a degradation in the quality of the listening experience. This problem has been addressed by using methods of enhancing the low frequency parts of an audio signal so that the quality of the music when played over small loudspeakers is improved.
A method for digitally modifying low frequency components of audio signals should satisfy the following design constraints;                a) music processed using the method should not sound unpleasant (constraint c1);        b) the method should not noticeably alter the tonal character of musical instruments (constraint c2);        c) the method should also not noticeably affect high frequency signals (constraint c3);        d) the method should not alter the balance of left and right channels of the stereo signal (constraint c4);        e) the method should not be prohibitively complex (constraint c5);        f) the method must not cause the maximum signal level of the digital system to be exceeded (constraint c6); and        g) the method must not amplify or introduce very low frequency signals that cannot be reproduced by small loudspeakers (constraint c7).        
In digital signal processing, it is essential that the maximum signal level (i.e., unity) of the audio signal is never exceeded, otherwise extreme audible artifacts can result. For example, considering a 16-bit audio signal, the range of signal values is −32,768 to +32,767 (i.e., 216 different values). If the signal value exceeds +32,767 then the signal value overflows to −32,768 rather than to its next highest value of +32,768. This creates extreme artifacts in the audio signal, and it is therefore vital to insure that the maximum signal limits are never exceeded. In analogue systems this type of signal overflow does not occur on account of the operating tolerances of devices such as valves, diodes, capacitors, transistors and resisters.
Low frequency audio signals may be enhanced using a number of techniques, some of which will now be discussed. One method of enhancing such signals is by the use of equalization wherein either one or many filters with different gain values process the signal, and lower frequency signals are given higher gain values. Thus low frequencies (and hence bass instruments) are emphasized by virtue of the fact that lower frequency energy is amplified. However, since most of the signal energy in a musical audio signal is concentrated into the lower frequencies this technique cannot amplify the low frequency signals very much before the maximum signal level in the system is exceeded, or the music sounds distorted. This method therefore breaks constraints c6 and c7.
Multi-band compression is another technique that may he used to enhance bass frequencies. In this technique one or many filters are arranged in parallel, as in equalization. The output of each filter is processed by a dynamic compressor that measures the current signal level in that frequency hand and applies a gain whose amount is related to that measurement. The signal energy in each frequency band is averaged over time and the compressors are arranged to operate such that the bass
frequency bands are kept at a higher average energy than the higher frequency bands. Bass instruments (such as bass drums, guitars etc) are therefore emphasized because the signal energy in higher on average in the lower frequency bands than in the higher frequency bands. However, since the gain of each band is independent of other bands, a musical instrument will often be noticeably tonally changed since the fundamental and harmonics of a note often falls across many frequency bands. This method therefore breaks constraint c2.
A device which uses a method similar to multi-band compression is disclosed in U.S. Pat. No. 5,359,665 (Aphex Systems Ltd). This device is an analogue device which combines phase inverted, dynamically compressed bass frequencies with the original audio signal. It provides greater enhancement of the bass frequencies when they are at lower levels and less enhancement when they are at higher levels, thereby satisfying constraint c6. However, the device is not designed to work with stereo signals: if the device is used for the left and right channels of a stereo signal, the musical image will drift from side to side as the left and right gain factors are independent of one another. This breaks constraint c4. The device also does not employ a very low frequency cut-off filter that would prevent the breaking of constraint c7. For very small loudspeakers, this device would not be able to reproduce maximum level signals.
Published International Patent Application No. WO-A1-9846044 (K. S. Waves Ltd) describes an apparatus and method for bass enhancement The method uses the psycho-acoustic principle of virtual pitch, in which the pitch of a musical note is recognized by the frequency spacing and relative level of the harmonics of the fundamental of that note. The method involves taking a certain range of bass frequencies and generating artificial harmonics of these frequencies which are at a higher frequency than the fundamental bass frequencies. The original bass frequencies are not actually present in the processed signal, only the artificial harmonics, as the fundamental does not need to be present for the pitch to be recognizable. However, audio signals which are processed using this method can be musically unpleasant as, for the effect to be noticeable, frequencies in the range 100 to 300 Hz must be quite loud. This breaks constraint c1. Also, the apparatus has many functional components and higher-order statistical processing which breaks constraint c5.